1. Field of the Invention
The present invention relates to a surface acoustic wave apparatus, and more particularly, to a surface acoustic wave apparatus having a balun function.
2. Description of the Related Art
Conventionally, there has been extremely remarkable progress made in relation to some techniques for making recent cellular phones compact in size and light in weight. In order to obtain such achievements, research activity has been carried out to reduce various components as well as the size of the required components. In fact, this research activity has also attempted to develop some components each having a plurality of functions.
In view of the above, there has been active research conducted in recent years to develop an apparatus having a so-called balun function incorporated in surface acoustic wave filters for use in an RF band of a cellular phone. Particularly, the balun function has been mainly utilized in a GSM system (Global System for Mobile Communication) and other such systems. Further, there is also an extremely high possibility that the balun function can be used in AMPS, PCS, DCS and other components. Here, “balun” means a circuit for matching a balanced line such as a parallel two-wire feeder with an unbalanced line such as a coaxial cable.
FIG. 12 is an explanatory view showing a widely used basic constitution of a surface acoustic wave apparatus which has such a balun function and having an input impedance and an output impedance which are substantially equal to each other. An arrangement shown in FIG. 12 represents a surface acoustic wave apparatus which includes a package 101, and a piezoelectric substrate 108 included in the package. Mounted on the electric substrate 108 are two stages of longitudinally connected surface acoustic wave filters 109 and 110, belonging to a longitudinally combined resonator type, each consisting of three comb-like electrode sections (which can also be called rattan blind-like electrodes, hereinafter referred to as an IDT (Interdigital Transducer)).
The first surface acoustic wave filter 109 is a longitudinally combined resonator type surface acoustic wave filter, which includes an IDT 112 and an IDT 113 arranged respectively on the right side and the left side of a central IDT 111 (along the propagating direction of a surface acoustic wave), also includes a reflector 114 and a reflector 115 which are arranged in a manner such that the above IDTs are interposed therebetween.
The second surface acoustic wave filter 110 is constructed in exactly the same manner as the first surface acoustic wave filter 109, thereby providing an arrangement in which the two longitudinally combined resonator type surface acoustic wave filters are longitudinally connected with each other. Here, a terminal 133 is an unbalanced signal terminal, while terminals 135 and 136 are balanced signal terminals. These terminals are electrically connected with the package 101 through metal wires. The unbalanced signal terminal 133 of the longitudinally combined resonator type surface acoustic wave filter 109 on an unbalanced side, as well as a ground terminal 134, are connected with unbalanced side electrodes 103 and 104 of the package 101. On the other hand, the respective balanced signal terminals 135 and 136 of the longitudinally combined resonator type surface acoustic wave filter 110 on the balanced side, are respectively connected with balanced side electrodes 105 and 107 of the package 101.
Accordingly, a surface acoustic wave filter having a balun function is required to have a propagating characteristic that in its pass bands between the unbalanced signal terminals and the balanced signal terminals, the amplitude characteristics are as equal to one another as possible, and that the phases are reversed by as close to 180 degrees as possible. In practice, these characteristics are respectively referred to as amplitude balance and phase balance.
With respect to the amplitude balance and the phase balance, it is accepted to regard each of the above-described surface acoustic wave filters having the balun function as a three-port device. For example, when the unbalanced input terminal is assumed to be a first port, the balanced output terminals are assumed to be second and third ports respectively, while the amplitude balance and the phase balance may be defined as follows:Amplitude balance=|A|, A=|20 log S21|−|20 log S3|;Phase balance=|B−180 |, B=|∠S21−∠S31|
Here, S21 and S31 are matrix elements when the three-port device is expressed in a scattered matrix, respectively representing a propagating characteristic between the first port and the second port, as well as a propagating characteristic between the third port and the first port. As to such balances, it is preferable that within the pass bands of the filter, an amplitude balance is 0 dB and a phase balance is 0 degree. On the other hand, when outside the pass bands, it is preferable that the amplitude balance is 0 dB and the phase balance is 180 degree.
However, in an arrangement shown in FIG. 12, since there is a deviation between an actual value and the aforementioned ideal value, it has been very difficult to achieve the desired level of characteristics. The reason for this may be explained as follows. The shapes of the balanced signal terminal 135 and the balanced signal terminal 136 are not symmetrical with respect to each other. Moreover, since the metal wires 140 and 142 for electrically connecting the package 101 are not symmetrical with each other either with respect to their shapes and lengths, a parasitic capacitance as well as a parasitic inductance in the respective balanced signal terminals 135 and 136 are different from each other. This means the presence of a problem that a balance, particularly a balance outside the pass bands will be deteriorated, hence undesirably reducing an attenuation outside the pass bands.